Patent Application
20130052479
This disclosure relates to a method of laser shock peening an object. More particularly, the disclosure relates to laser shock peening a 7000-series aluminum structure, such as a fan blade.
The airfoil section of a fan blade must withstand high cycle, notched fatigue- type loading, often resulting from scratches or dents from a variety of foreign object damage mechanisms. In general, aluminum alloys exhibit relatively low notched fatigue strengths. A common method of improving high cycle fatigue strength of a variety of metals and alloys is through shot peening which imparts compressive residual stresses near the surface. However, residual stresses generated by conventional shot peening methods are confined to a depth of less than 0.20 mm (0.008 inch) from the surface. Furthermore, conventional shot peening introduces significant amount of cold work in the surface zone that may lead to reduced ductility and toughness.
Laser shock peening (LSP) is a surface treatment process designed to improve the mechanical properties and fatigue performance of materials. LSP uses a high intensity laser and an overlay to generate high pressure shock waves on the surface of the object. An increase in fatigue strength is accomplished by the creation of large magnitudes of compressive residual stresses and increased hardness which develop in the subsurface. The maximum compressive residual stress is often formed at the surface of the object and decreases in magnitude with increasing depth below the surface. The transient shock waves can also induce microstructure changes near the surface and cause a high density of dislocations to be formed. The combined effect of the microstructure changes and dislocation entanglement contribute to an increase in the mechanical properties near the surface.
Laser shock peening has been used to strengthen airfoils, such as turbine engine fan blades constructed from titanium or nickel. Laser shock peening processes have not yet been developed for use with aluminum airfoils, such as fan blades.
Disclosed is a method of laser shock peening an aluminum alloy fan airfoil to improve its resistance to failure by notched fatigue. In one example, the airfoil is made from 7255 aluminum alloy. The laser has a power density of at least 10 GW/cm2 (220×109 BTU/hr·in2) and a pulse width of <50 ns to produce a shock peened layer extending a depth of 0.030-0.040 inch (0.8-1.0 mm) beneath the object surface.
The disclosure can be further understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
A method of laser shock peening an object is illustrated in
Returning to
In one example, the object surface 14 is also submerged in a transparent media or tamping material 20, such as water, so that the rapidly expanding plasma 30 cannot escape and the resulting shock wave 32 is transmitted into the object's subsurface. These shock waves 32 can be much larger than the dynamic yield strength of the material (>1 GPa or >145 kpsi) and cause plastic deformation to the object surface 14 and compressive residual stresses which can extend a depth 34 (for example, 0.030-0.040 inch (0.8-1.0 mm)) beneath the object surface 14 into the subsurface. Because of the high strains/strain rates that the object 12 undergoes, there can be significant microstructure changes that can result in changes in the mechanical properties of the affected region.
In thin materials like blade edges, the laser peening shock pressure can be intense as it reaches a backside 22 of the blade opposite the object surface 14. An acoustic matched backer material 24 can be used to support the object 12 and couple out this pressure wave so as not to allow it to reflect as an undesired tensile wave. If the blade is thick enough, then the shock pressure will have sufficiently attenuated at the point of reaching the backside 22 such that it no longer yields the material and does not need to be coupled out.
Multiple LSP passes may be employed to achieve complete surface coverage and create desired residual stress profiles. LSP results in virtually unaltered surface finish of the finish machined components and limited transient heating effects, whereas the same is not true of shot peening methods. However, it should be understood that both shot and shock peening can be used on the same object in either the same or different locations.
Multiple LSP parameters were evaluated as a guide to achieving desired fatigue performance for a 7000-series aluminum, in particular a 7×55 aluminum alloy. Specimens in a baseline (as-machined) condition as well as specimens laser shock peened with various parameter combinations were tested for residual stress. The LSP parameters are denoted by the shorthand nomenclature X-Y-Z where X is the power density, Y the laser pulse width, and Z the number of layers of full laser peening coverage (i.e. 4-18-2 is 4 GW/cm2, 18 nanoseconds pulse duration, and 2 layers coverage). If the object surface 14 will be laser shock peened more than once, typically the ablative coating is reapplied to the object surface 14 and re-immersed into the tamping material 20. The laser spot overlap is 50%, in one example. In one example, the laser shape is square, although any suitable shape may be used.
Initial experiments showed that residual stress profiles and magnitudes were not strongly dependent on the LSP parameters. The parameters of 4-18-2 provided good results (an increase in fatigue life of at least about tenfold for a constant stress), although other values within the specified range may be used.
Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.
